Just over 30 years ago, computed tomography (CT) was heralded as a remarkable technology that would change the practice of medicine. CT has certainly lived up to this billing and has continued to be an invaluable tool for imaging evaluation across all ages. Applications of CT are extensive, including its use as a primary imaging modality, a problem-solving tool, and even recently in a screening capacity. In recent years, advances in CT technology, such as the introduction of helical and subsequently multidetector CT (MDCT), have resulted in a dramatic increase in the number of CT studies performed.
is a Resident in Radiology, Beth Israel Deaconess Medical Center,
is a Professor of Radiology, Division of Pediatric Radiology,
Department of Radiology, Duke University Medical Center, Durham,
2006;(Vol. 35, No. 4):13-20.
Just over 30 years ago, computed tomography (CT) was heralded as
a remarkable technology that would change the practice of medicine.
CT has certainly lived up to this billing and has continued to be
an invaluable tool for imaging evaluation across all ages.
Applications of CT are extensive, including its use as a primary
imaging modality, a problem-solving tool, and even recently in a
screening capacity. In recent years, advances in CT technology,
such as the introduction of helical and subsequently multidetector
CT (MDCT), have resulted in a dramatic increase in the number of CT
be used in a great many situations, the question remains whether CT
be used in these situations. Moreover, when CT is indicated, one
must also consider how the CT is to be performed. These questions
began to be raised globally only a few years ago following
publications in both scientific literature and public media.
The principal thrust of this material was directly or indirectly
addressing the amount of radiation from CT and the potential risks
of cancer. Following a series of articles in the
American Journal of Roentgenology
, and the ensuing media attention, this has continued to be a
sentinel issue that involves radiologists, manufacturers, and
regulatory agencies alike. Although healthcare professionals agree
that the benefits derived from CT imaging studies must outweigh its
risks, it takes thought and effort to achieve this balance. This
effort is particularly important when considering CT scans of
infants and children in whom CT techniques are less well studied
and understood, and the potential risks of radiation exposure are
To best reach this objective, there must be an understanding of
the use of CT, a comprehension of the amount of radiation that CT
delivers and the appropriate measure of that exposure, discussion
and debate about low-level radiation and cancer development, and a
consideration of the methods used to manage the amount of radiation
from CT. Therefore, the purpose of this review is to provide
salient information for diagnostic radiologists regarding use of
pediatric CT, the relevant risks, and strategies to manage the
Understanding the language: Measures of CT
In order to build a foundation for understanding radiation dose
and its potential risks, it is important to be familiar with
pertinent radiation measures and terminology for CT. Among the most
frequently used terms are
absorbed dose, effective dose equivalent
indices of dose;
these measurements are determined from phantom measurements,
consisting of the
CT dose index
dose length product
(DLP). Absorbed dose, measured in grays (Gy), represents the actual
radiation dose delivered to a specific tissue or organ, such as the
breast, thyroid, or bone marrow. While absorbed (or organ) dose
allows us to determine biologic effects and risks, it is cumbersome
for routine assessment of CT radiation. One would need to either
estimate doses based on CT parameters and use mathematical models,
or make direct measurements, which is impractical, at best. The
dose equivalent, measured in sieverts (Sv), is the product of the
absorbed dose and a quality factor.
The quality factor depends on the type of radiation. For
example, for CT and other X-ray studies, the factor is 1.0. The
dose equivalent is used to calculate the effective dose equivalent
(EDE). The EDE is the sum of the product of each dose equivalent
and a weighting factor for susceptibility to radiation for an
individual tissue or organ. The EDE is the equivalent dose to the
whole body resulting from general or regional exposure. The EDE
provides a useful method to compare doses from different regional
radiation exposures (such as brain CT versus ankle CT), and between
different imaging modalities (such as radiography, scintigraphy, or
CT). Despite the usefulness of EDE, there are limitations. At best,
effective dose equivalent is still an estimate for an individual
patient. The EDE is typically derived either from mathematical
patient models or anthropomorphic phantoms, or by using the CT dose
index (CTDI) or dose length product (DLP) (see below) and
There are also measures of radiation that are available on CT
scanner consoles. The CTDI is measured in units of mGy and
represents the radiation dose in a single CT slice determined using
acrylic cylinder phantoms of a standard length, and, in general, 2
diameters: 16.0 and
32.0 cm. An in-depth discussion of the various and often
confusing methods for determining CTDI is beyond the scope of this
review and can be found elsewhere.
The volume CTDI (CTDI
), is currently the preferred measure and takes pitch into account.
The dose length product (DLP) is measured in units of mGy·cm. The
DLP, in simplified terms, is the product of scan length and CTDI
. It makes sense that a longer area of coverage will provide a
higher dose than a shorter area. This is why CTDI
itself is of no practical use in estimating dose. It is only a
measurement at a single slice. Thus, the DLP represents a better
method for estimating dose (that is, the effective dose
equivalent), since a larger area of coverage will have a larger DLP
than a smaller area of coverage, using the same CT parameters.
Contemporary MDCT scanners can display (although may not record)
the CTDI and DLP. This serves an important role, as it allows
technologists and radiologists an opportunity (if recognized) to
assess how individual CT settings will affect dose for that
particular scan or protocol. For example, while the actual dose to
the patient is not known, selection of parameters that result in a
50% decrease in the DLP will generally result in a 50% reduction in
dose to that patient.
Trends and patterns: How do we use CT?
While the precise number of CT examinations performed in the
United States each year is unknown, CT is a frequently performed
examination. For example, it has been estimated that up to
65,000,000 CT examinations are performed in the United States each
This equates 1 CT performed for every 4 to 5 people, based on
recent census numbers. In addition, trends show a growth of CT use
in recent years. Estimates are approximately a 10% growth in use
It is important to recognize that these estimates are several years
old and do not include the newest trends and applications of MDCT,
especially screening CT. Moreover, numbers do not take into account
multiphase examinations, where each phase generally provides what
we refer to as a radiation "dose event." That is, a 3-phase abdomen
CT, which is considered 1 exam, actually provides up to 3
examinations' worth of radiation-1 for each phase (if there are no
adjustments in techniques between phases). If more contemporary
practice patterns were available, and nuances such as the frequency
of multiphase examinations were better understood, it would not be
unreasonable to expect that the amount of radiation delivered to
the population from CT is increasing at an exponential rate rather
than a linear rate.
If we apply some of the above numbers to pediatric CT, a
population at higher risk from radiation, one can also appreciate
that CT use in this population is not nominal. For example, it has
been estimated that up to 11% of CT scans performed each year are
on infants and children.
If there are 65 million examinations in the United States per year,
this leads to the conclusion that as many as 7.1 million pediatric
CT scans are being performed annually. These numbers are
substantially more than the estimates of 600,000 to 1.3 million
pediatric exams per year in the United States cited in a recent
As with adults, it is also reasonable to assume that pediatric CT
use is increasing in a nonlinear fashion, given the applications
that have found favor, such as the evaluation of the common
disorders in children, particularly appendicitis.
It is also important to realize that CT contributes a
disproportionate amount of medical radiation and, therefore,
overall radiation to the population. Background sources typically
account for 80% to 85% of all radiation exposure. Medical exposure
accounts for approximately 14% of the remainder. Of this medical
exposure, CT accounts for up to 67% of the total, despite the fact
that CT examinations accounted for only 5% to 11% of X-ray
Is the relative radiation dose from CT studies recognized? In a
recent informal survey of approximately 50 pediatricians, the
majority believed that, given its frequency, radiography, not CT,
was the greatest contribution to pediatric radiation dose
(unpublished data, DP Frush from the American Academy of Pediatrics
Meeting, New Orleans, LA, 2003). This supports a recent article in
which investigators surveyed radiologists, emergency department
patients, and physicians.
While it may not be surprising that 100% of patients and 73% of
emergency department physicians underestimated the radiation dose
from an abdominal CT, 78% of radiologists underestimated the dose,
with 5% of radiologists surveyed answering that a CT provided equal
to or less than the amount of radiation from a chest radiograph.
This underscores the importance of continued education of patients
and healthcare workers alike.
How do CT doses compare with other modalities? First, the dose
of radiation delivered by CT ranges from approximately 1.0 mSv to
30 mSv per phase, not necessarily per examination.
In comparison, background radiation exposure is typically roughly
3.0 mSv per year. Chest radiography is approximately 0.03 mSv (less
than 1/100th the dose). Therefore, after background radiation, CT
provides the single largest source of radiation exposure.
Is the radiation from CT harmful?
A CT examination delivers a radiation dose below 100 to 150 mSv,
which is the upper limits of the range considered low-level
There is little argument that doses in excess of 200 mSv have a
significant risk for the development of cancer. The question is
whether the radiation from CT causes cancer. The most defensible
answer is no: There is no direct proof that the radiation from CT
results in cancer. To investigate this formally would be
tremendously costly, primarily because of the large size of the
population and the decades of observation that would be needed.
What we are left to assess is data from other low-level
The connection between low-level radiation exposure, such as
that from CT, and cancer is debated. Even recently, the arguments
for and against radiation risks have been juxtaposed.
Sources for more in-depth discussion of risks are also available.
One review contesting the association of cancer and low-level
radiation discusses hormesis.
Hormesis is the perspective that low-level radiation exposure has a
beneficial effect. This viewpoint finds support in studies of the
immune system as well as epidemiologic data that conclude that the
occurrence of cancer is lower in low-level exposure than in
On the other hand, data can easily be found that demonstrate that
radiation doses in the range of those found in CT are associated
with a significant increased risk of fatal cancer. In a recent
review on the issue of low-level radiation and cancer, authors
note, "Above doses of 50 to 100 mSv (protracted exposure) or 10 to
50 mSv (acute exposure), direct epidemiologic evidence from human
populations demonstrate the exposure to ionizing radiation
increases the risk of some cancer."
While proponents do point out that this is only a small percent
above background, given the frequent and increasing use of CT, this
is still considered a potential public health issue.
Of note, 2 recent articles have addressed the issue of
bioeffects and radiation. In the first, published in the
British Medical Journal
, investigators noted cognitive deficits and a decrease in high
school attendance in adolescent boys who received head and neck
radiation for cutaneous malformations as young children.
In the second article, published in
, estimates of cancer mortality from X-ray studies was discussed,
and estimates of attributable risk ranged from 0.6% to 3%.
The authors conclude that, in the United Kingdom, this results in
approximately 700 additional cancers from radiologic studies. While
it is not the intent of the present article to critique these
investigations, these articles underscore the ongoing attention to
the issue of low-level radiation and potential bioeffects.
In spite of these arguments, virtually all experts agree that
excessive and unnecessary radiation should be avoided. This is
particularly true in infants and children for several reasons: This
group has a longer lifespan in which to manifest radiation-induced
cancers; their organs and tissues are more susceptible to
radiation; and similar radiation exposures to those used in adults
can result in a greater dose in children.
In effect, the debate over the development of cancer and low-level
radiation could be simplified by ascribing to the practice that has
been the standard for radiologists for decades: Unnecessary
radiation should be minimized or eliminated-the ALARA (As Low As
Reasonably Achievable) principle.
How can radiologists manage CT dose?
The first step to limiting CT dose is perhaps the most obvious,
but one of the easiest to overlook. As the number of requests for
CT examinations continues to rise, radiologists have a
responsibility to perform only those studies that are justified.
While not always possible or practical, we should not perform a
questionable examination because it is easier than trying not to do
it-a not unfamiliar position. It has been noted that, similar to
other modalities, perhaps >30% of all CT examinations performed
on children are not clearly indicated.
Because of the increasing use at all ages, it is reasonable to
expect that the number of CT examinations ordered for questionable
indications will increase as well.
While determining the appropriateness of every CT requested is
impractical at best, there is a successful (although often
time-consuming) method to minimize the number of unnecessary
studies. This method is communication between the ordering
physician and radiologist. Communication can help to ensure that a
different imaging modality, such as sonography or MR imaging, may
be substituted for the CT, when appropriate. Along these lines,
communication in the form of conferences or lectures on a local,
national, and international level to healthcare providers, such as
family practitioners, pediatricians, and emergency physicians, can
address both the issues of CT benefits and appropriate applications
as well as risks.
Once it has been agreed that CT is the study of choice, the
radiology team also has a responsibility to apply CT protocols or
techniques that are adjusted based on various considerations. These
considerations include indication for the scan, region scanned, and
size of the patient. The CT technical parameters can be adjusted
based on indication. While individual parameter adjustments for the
common applications of CT have not been completely addressed, in
reductions in tube current
should be considered in situations in which the abnormality will be
large or will exhibit high contrast. For example, evaluation of
renal calculi can be performed at a lower tube current because of
the high contrast nature of stones.
Scan settings should also be appropriate for different regions of
the body. It has been recognized that chest CT for evaluation of
lung parenchyma can be performed at a lower tube current, reducing
Skeletal CT also lends itself well to reductions in tube current,
given the high intrinsic contrast of bone versus soft tissue.
Finally, scan parameters need to be selected based on the size of
This is a necessity in pediatric CT, given that patient weight can
vary by a factor of more than 100.
To make these adjustments in scan techniques, it is necessary to
have a basic understanding of the contributions to image quality
and radiation dose for those parameters that are selected for
protocols or individual examinations. Guidelines for parameters for
pediatric body CT are found in Tables 1 through 4. These parameters
consist of tube current (milliampere-mA), gantry rotation time
(seconds), kilovoltage (kVp), pitch, table speed (mm per rotation),
and detector configuration (number of detector rows and detector
thickness). CT technologists should also have an understanding of
these parameters and their effects, since these individuals will
often be the first and only determinants of techniques.
Tube current determines the amount of photons delivered by the
X-ray tube. The higher the tube current, the greater the number of
photons and the less the image noise, or mottle. Adjusting the tube
current should be an obvious starting point for reducing the amount
of radiation delivered to children. Because of their smaller size,
children do not need to be exposed to the same tube current as do
adults. While decreasing tube current will decrease scan quality,
studies based on a simulated tube current reduction technique
are being conducted that determine detection thresholds for mA for
several different scan indications such as appendicitis and renal
calculi (Figure 1).
Gantry cycle time will also affect the amount of radiation
delivered by a CT scan. Holding all other parameters constant,
faster cycle time will result in a proportionate decrease in
radiation. For example, by reducing the cycle time from 1.0 to 0.5
seconds, the amount of radiation delivered is decreased by 50%. In
general, we use the fastest cycle times in children for several
reasons. The motion from the heart and lungs (in non-breathhold
examinations) will be decreased. In addition, the faster the
examination can be completed in children the better, since they may
have limited ability to hold still or hold their breath. This
means, for a 75-mA examination, we would use 150 mA at 0.5-second
rotation time instead of a 1.0-second rotation time and 75 mA.
Peak kilovoltage (kVp) determines the energy of the photons. In
addition to affecting image contrast, kVp also plays a role in
determining radiation dose. Decreasing kVp will increase contrast,
increase noise, and decrease radiation dose. Unlike with tube
current and gantry cycle time, the relationship between dose and
kVP is not linear, but exponential. This means that incremental
decreases in kVp result in more substantial decreases in dose. For
example, going from 140 to 120 kVp can reduce the radiation dose by
approximately 35%. Although there is currently little conclusive
data on the effect of kilovoltage adjustments on diagnostic
quality, recent attention is being directed to the relationship
between kilovoltage, image quality, and radiation. In a recent
investigation, the authors concluded that weight-adjusted
contrast-enhanced chest CT could be routinely performed at 80 kVp.
Our experience is that kVp <120 is acceptable in young children,
including CT angiography (Table 1). To date, however, there has
been little investigation of reductions in kVp in pediatric CT.
Pitch is another factor that contributes to radiation dose. In
general, we have found no benefit to performing an MDCT at pitches
<<1.0 except in very young children. In this group, more so
than in larger children, our experience has been that the streak
artifact at interfaces with a large difference in attenuation (such
as air-filled bowel and liver) is more troublesome as pitch
increases. In very young children, we will generally resort to a
pitch just under 1.0 (Table 1). For CT angiography, higher pitches
(with narrow detector width for thinner slice acquisition and
improved rendering) provide excellent quality (Table 1).
Increasing the detector width (eg, from 0.625 to 1.25 with the
GE 16-slice scanner) while keeping other parameters constant will
also result in a lower dose. However, the ability to obtain thinner
slices will be limited. The selection of these parameters will,
therefore, need to balance these factors. For general body CT in
children, detector thicknesses of 1.25 mm for 16- and 8-slice, and
2.5 mm for 4-slice examinations (LightSpeed, GE Healthcare,
Waukesha, WI) are usually adequate. Thinner detector widths can be
selected when high-resolution multiplanar and 3-dimensional
reconstructions are anticipated, such as with CT angiography,
skeletal assessment, and airway evaluation.
There are a few other strategies to consider as well. Multiphase
examinations should be limited. In general, each phase will add to
the overall radiation dose. There is little to no benefit for
routine use of multiphase body CT in children. When additional
phases are necessary (for example, precontrast assessment or
excretory phase for the kidneys), adjustments in above parameters
such as larger pitch, decreased kVp, or decreased mA should be
considered. Finally, the use of shields for pediatric chest CT has
also been addressed. While no pediatric shield is commercially
available at this point in time, we have found that there can be
nearly a 30% reduction in breast dose in infants with the use of a
Recent advancements and future directions in CT radiation
Efforts in reducing CT dose and balancing the need to reduce
radiation dose against image quality include the use of age- or
weight-adjusted CT protocols,
application of automated tube current modulation (ATCM), a form of
automatic exposure control,
and research assessing the effect of dose reduction on image
quality and lesion detection.
Age- or size-adjusted protocols for pediatric scanning have been
increasingly available from the major CT manufacturers. Industry
guidelines for pediatric CT were rare up to just a few years ago.
Manufacturers are now addressing the unique concerns associated
with pediatric CT.
Automatic tube current modulation is composed of 2 basic
techniques: Angular modulation (where adjustments are based on the
elliptical cross-sectional geometry of the body) and z-axis
modulation (where mA is modulated based on regional attenuation of
the X-ray beam). Automatic tube current modulation technology
varies among manufacturers, but the objective is the same. Tube
current is modulated based on a desired level of image noise. The
amount of noise varies depending on several patient
characteristics, such as geometry, overall circumference, and
cross-sectional densities. For example, mA is increased when
scanning the upper chest including the shoulders versus the upper
mediastinum, or in the narrower anteroposterior dimension versus
the mediolateral dimension during rotation of the gantry. This is
in contrast to standard technology in which one tube current is
selected for an entire region. One recent review has noted several
studies in which the use of ATCM has shown radiation dose
reductions (without loss in diagnostic quality) ranging from 10% to
Tube current reductions and their effect on study quality is
also being addressed systematically (using a research tool) in
adult and pediatric CT.
With this technology, noise is added to the CT projection data set
to simulate lower tube current. Initial assessment on study quality
suggested that reductions of up to 67% were possible for pediatric
body MDCT. Further efforts with simulated tube current reduction
include assessment of dose reduction and diagnostic quality for
pediatric head and neck CT.
We examined both water phantoms and skull phantoms using a
16-slice MDCT with parameters identical to those used in routine
institutional craniofacial MDCT studies. Noise and anatomic detail
for the skull phantom were evaluated. The conspicuity of anatomic
regions in the simulation versus those in the actual reduced-mA
examinations was determined by consensus evaluation of 3
neuroradiologists who were blinded to the mA of the scan. Most
importantly, review of the images by 3 faculty observers
demonstrated no difference in the conspicuity of major bony
anatomical structures (Figure 2; unpublished data, S. Mukundan,
Duke University Medical Center, Durham, NC, 2004). The simulated
tube current reduction tool is currently being applied to head CT
obtained to assess for ventriculoperitoneal shunt malfunction as
well as sinus disease.
CT is a powerful imaging tool. With the technologic improvements
of MDCT, the number of CT scans being ordered is rapidly
increasing. However, for all of its benefits, there are potential
risks to the injudicious use of CT. Over the past few years,
attention has been focused on an acknowledged, but often
underappreciated, risk of CT use in the radiation dose and
potential cancer risk. In order to address the issue of CT and
radiation, radiologists must have a working understanding of the
widespread and increasing use of the modality, the basic measures
of CT radiation, the potential risks of radiation exposure, and the
strategies for management. With this understanding, radiologists
can continue to educate clinical colleagues and provide a service
for patients that is informed and responsible.